Environmental science

Covid-19: New data shows ‘growing gap’ between reported cases and Canterbury wastewater levels

The Canterbury regional health authority fears up to half of Covid-19 cases are going unreported, as new data shows a growing gap between cases and virus fragments in wastewater.

On Friday, the Crown research institute of Environmental Science and Research (ESR) launched an interactive Covid-19 wastewater dashboard to share how the virus was tracking across New Zealand.

The dashboard lets users check how much Covid-19 there is in wastewater in different regions, compare it with reported cases, and see if levels are increasing or decreasing over time.

ESR science leader Brent Gilpin said one major trend they were noticing was a “growing gap” between reported cases and the amount of Covid-19 virus in wastewater.

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On Monday interim Waitaha/Canterbury district director for Te Whatu Ora (Health NZ) expressed concern in their newsletter that they might be underestimating current Covid-19 cases by up to half, based on the new wastewater data.

This came as the department revealed there were 169 people in Canterbury’s hospitals with the virus – the highest number since the pandemic began.

That meant one in five of roughly 836 people currently hospitalized with Covid-19 were in Canterbury, despite the district only making up about one-tenth of the population.

However, when reported Covid-19 cases and wastewater levels were compared on a national level, the numbers were quite similar.

The national trend for Covid-19 reported case numbers vs virus particles in wastewater.

ESR

The national trend for Covid-19 reported case numbers vs virus particles in wastewater.

Gilpin believed wastewater data would eventually be an accurate measure of Covid-19 in the community, similar to how it was used to measure drug use.

“Over time, less and less people will use rapid antigen tests,” he said.

Gilpin said to get the data, they collected wastewater samples from different catchments and extracted viral RNA from it.

They then used that to calculate genome copies per liter of wastewater, multiplied by liters per day going into that catchment, divided by how many people lived there.

But accurately measuring how viral RNA levels in wastewater equated to actual cases was still a work in progress.

People could shed billions of particles or they could shed very few, he said, and it often varied over the course of their illness.

ESR has been testing wastewater samples across the country for the Covid-19 virus.  (File photo)

ESR/Supplied

ESR has been testing wastewater samples across the country for the Covid-19 virus. (File photo)

It was something Kiwi researchers were working on with their overseas counterparts to understand better.

Gilpin said they could also see which variants of the virus were spreading most widely in communities.

Five weeks ago, Omicron subvariant BA.2 was the most common in the country, while subvariant BA.4/5 made up about 10% of cases. Now it made up 75%, he said.

The amount of Covid-19 in wastewater started shooting up around February, in line with Omicron’s exponential spread across New Zealand – with case numbers topping 20,000 daily at the wave’s peak.

Canterbury's trend for Covid-19 reported case numbers vs virus particles in wastewater.

ESR

Canterbury’s trend for Covid-19 reported case numbers vs virus particles in wastewater.

Wastewater levels had peaked around March and Gilpin said that, just like reported case numbers, they had dropped slightly since.

“It has not dropped quite as much as I guess we had hoped … as we go forward hopefully we will see the levels in wastewater start to drop.

“The levels in Auckland peaked much earlier than the rest of the country, because Covid was circulating there much earlier.”

While they were seeing levels in the South Island peak a little later, they had converged across most of the country.

In central Auckland, genome copies per person, per day were slightly lower than some regions, he said, but “certainly not the difference seen last year”.

Auckland's trend for Covid-19 reported case numbers vs virus particles in wastewater.

ESR

Auckland’s trend for Covid-19 reported case numbers vs virus particles in wastewater.

Gilpin said the wastewater data could provide confidence that any drop in Covid-19 cases was genuine, rather than just the result of fewer people testing.

This was just the first version of the wastewater tool and Gilpin said they hoped to eventually be able to use it to provide more in-depth data on potentially even more infectious diseases.

It was important to make sure the data itself was out there for the public and fellow researchers alike to use and analyze, he said.

“This is the way public health has to progress … so people can make their own decisions.”

As well as crowdsourcing a combined truth, Gilpin said it could help “overcome the challenges of disinformation itself”.

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